Process for minimizing benzene, toluene, and a recycle loop in a zero benzene aromatics complex

ABSTRACT

The present invention relates to minimizing benzene, toluene, and an A9/A10 recycle loop in a zero benzene aromatics complex. More specifically, the present invention relates to a minimizing benzene, toluene, and an A9/A10 recycle loop in a zero benzene aromatics complex wherein the aromatic feed has a low methyl to phenyl ratio, and where the aromatic feed has a high methyl to phenyl ratio.

FIELD

The present invention relates to minimizing benzene, toluene, and anA9/A10 recycle loop in a zero benzene aromatics complex. Morespecifically, the present invention relates to minimizing benzene,toluene, and an A9/A10 recycle loop in a zero benzene aromatics complexwherein the aromatic feed has a low methyl to phenyl ratio, and wherethe aromatic feed has a high methyl to phenyl ratio.

BACKGROUND

Zero-benzene aromatics complexes are attractive to purified terephthalicacid producers because more of the feedstock is converted to the desiredproduct (para-xylene) as opposed to a side product (benzene). Toluenemethylation technology enables zero-benzene aromatics complexes. Thecurrent flow scheme converts all the toluene in a toluene methylationunit, and all the benzene and A9/A10 aromatics in a trans-alkylationunit. In the current flow scheme, the trans-alkylation unit convertsbenzene and A9/A10 aromatics to a trans-alkylation product comprisingbenzene through A11+ aromatics with toluene and xylenes as the focus.The problem with the current flow scheme is that some feed cases requirevery large recycle loops through fractionation columns in order toconvert the benzene and A9/A10 aromatics to extinction. These recycleloops require high costs. This is particularly the case when there isextra benzene in the feed (e.g., an aromatics complex with an externalbenzene feed or integrated with benzene producing technologies such asreforming or dehydrocyclodimerization) or when there is extra A9/A10material in the feed or produced in one of the aromatics complex units(e.g., toluene methylation).

In one embodiment, the disclosure addresses a case where extra benzeneand toluene are produced or brought into the existing aromatics complex.The stoichiometry of benzene and toluene relative to A9+ in the feed issignificantly below methyl to phenyl ratio of 1.5 in the current flowscheme. This leads to low per-pass conversion of benzene in thetrans-alkylation unit, resulting in very high benzene recycle. Thissituation would call for more efficient and balanced methylation vs.trans-alkylation unit operations to minimize the benzene and toluenerecycle. In this disclosure, at least a portion of the benzene isdiverted from the trans-alkylation unit and sent to the toluenemethylation unit. Benzene reactivity is shown to be at least half of thetoluene reactivity in a toluene methylation unit.

In another embodiment, the disclosure addresses a case where extra A9+materials are produced or brought into the existing aromatics complex.The stoichiometry of benzene and toluene relative to A9+ issignificantly above methyl to phenyl ratio of 1.5 in the current flowscheme. This leads to low per-pass conversion of A9+ in thetrans-alkylation unit, resulting in very high A9+ recycle. Thissituation would call for more efficient and balanced methylation vs.trans-alkylation unit operations to minimize the A9+ recycle. In thisdisclosure, at least a portion of the toluene is diverted from thetoluene methylation unit and sent to the trans-alkylation unit. Thetrans-alkylation unit converts benzene, toluene, and A9/A10 aromatics athigh per-pass conversion to a trans-alkylation product comprisingbenzene through A11+ aromatics with toluene and xylenes as the focus.

SUMMARY

A first embodiment of the invention is a process for producing nobenzene in an aromatics complex, comprising passing an aromatics streamhaving a low methyl to phenyl ratio to a benzene column to produce abenzene column overhead stream and a benzene column bottoms stream;passing the first portion of benzene column overhead stream to atrans-alkylation unit of A9+ with benzene to produce a trans-alkylationproduct stream and passing the benzene column bottoms stream to atoluene column which produces a toluene column overhead stream and atoluene column bottoms stream; passing the second portion of benzenecolumn overhead to methylation unit to produce methylated productstream; passing the toluene column overhead stream to a methylation unitto produce a methylation product stream; and passing the toluene columnbottoms stream to a fractionation zone to produce an overhead stream, abottoms stream, and a middle boiling fraction, wherein the middleboiling fraction is sent to the trans-alkylation unit of A9+ withbenzene.

A second embodiment of the invention is a process for producing nobenzene in an aromatics complex, comprising passing an aromatics streamhaving a methyl to phenyl ratio of about 0 to about 1.5 to a benzenecolumn to produce a benzene column overhead stream and a benzene columnbottoms stream; passing the first portion of benzene column overheadstream to a trans-alkylation unit of A9+ with benzene to produce atrans-alkylation product stream and passing the benzene column bottomsstream to a toluene column which produces a toluene column overheadstream and a toluene column bottoms stream; passing the second portionof benzene column overhead to methylation unit to produce methylatedproduct stream; passing the toluene column overhead stream to amethylation unit to produce a methylation product stream; and passingthe toluene column bottoms stream to a fractionation zone to produce anoverhead stream, a bottoms stream, and a middle boiling fraction,wherein the middle boiling fraction is sent to the transalkylation unitof A9+ with benzene.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by weight, unless otherwise indicated.Other objects, advantages and applications of the present invention willbecome apparent to those skilled in the art from the following detaileddescription and drawings. Additional objects, advantages and novelfeatures of the examples will be set forth in part in the descriptionwhich follows, and in part will become apparent to those skilled in theart upon examination of the following description and the accompanyingdrawings or may be learned by production or operation of the examples.The objects and advantages of the concepts may be realized and attainedby means of the methodologies, instrumentalities and combinationsparticularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the current design of the zero-benzene aromaticsprocess.

FIG. 2 illustrates the high methyl to phenyl ratio zero-benzenearomatics process.

FIG. 3 illustrates the low methyl to phenyl ratio zero-benzene aromaticsprocess.

DEFINITIONS

As used herein, the term “stream” can include various hydrocarbonmolecules and other substances.

As used herein, the term “stream”, “feed”, “product”, “part” or“portion” can include various hydrocarbon molecules, such asstraight-chain and branched alkanes, naphthenes, alkenes, alkadienes,and alkynes, and optionally other substances, such as gases, e.g.,hydrogen, or impurities, such as heavy metals, and sulfur and nitrogencompounds. Each of the above may also include aromatic and non-aromatichydrocarbons.

As used herein, the term “overhead stream” can mean a stream withdrawnat or near a top of a vessel, such as a column.

As used herein, the term “bottoms stream” can mean a stream withdrawn ator near a bottom of a vessel, such as a column.

Hydrocarbon molecules may be abbreviated C1, C2, C3, Cn where “n”represents the number of carbon atoms in the one or more hydrocarbonmolecules or the abbreviation may be used as an adjective for, e.g.,non-aromatics or compounds. Similarly, aromatic compounds may beabbreviated A6, A7, A8, An where “n” represents the number of carbonatoms in the one or more aromatic molecules. Furthermore, a superscript“+” or “−” may be used with an abbreviated one or more hydrocarbonsnotation, e.g., C3+ or C3−, which is inclusive of the abbreviated one ormore hydrocarbons. As an example, the abbreviation “C3+” means one ormore hydrocarbon molecules of three or more carbon atoms.

As used herein, the term “unit” can refer to an area including one ormore equipment items and/or one or more sub-zones. Equipment items caninclude, but are not limited to, one or more reactors or reactorvessels, separation vessels, distillation towers, heaters, exchangers,pipes, pumps, compressors, and controllers. Additionally, an equipmentitem, such as a reactor, dryer, or vessel, can further include one ormore zones or sub-zones.

The term “column” means a distillation column or columns for separatingone or more components of different volatilities. Unless otherwiseindicated, each column includes a condenser on an overhead of the columnto condense and reflux a portion of an overhead stream back to the topof the column and a reboiler at a bottom of the column to vaporize andsend a portion of a bottoms stream back to the bottom of the column.Feeds to the columns may be preheated. The top or overhead pressure isthe pressure of the overhead vapor at the vapor outlet of the column.The bottom temperature is the liquid bottom outlet temperature. Overheadlines and bottoms lines refer to the net lines from the columndownstream of any reflux or reboil to the column unless otherwise shown.Stripping columns omit a reboiler at a bottom of the column and insteadprovide heating requirements and separation impetus from a fluidizedinert media such as steam.

As depicted, process flow lines in the drawings can be referred tointerchangeably as, e.g., lines, pipes, feeds, gases, products,discharges, parts, portions, or streams.

The term “passing” means that the material passes from a conduit orvessel to an object.

As used herein, the term “toluene methylation” may also be used forbenzene methylation.

As used herein, the term “methyl to phenyl ratio” can be calculated asfollows:

Methyl:Phenyl Mole Ratio=[Total number of methyls]/[Total AromaticRings]

Where: Total Aromatic Rings=sum over all i (MS(i)/MW(i)*NR(i))

-   -   Total Number of Methyls=sum over all i (MS(i)/MW(i)*ME(i))    -   i: Compound Species    -   Molecular weight for species i: MW(i)    -   Number of aromatic (phenyl) rings for species i: NR(i)    -   Number of methyl groups attached onto the phenyl rings of        species i: ME(i)    -   The mass content of species i, in the feed: MS(i)        Exemplary calculations for various compound species are depicted        below:        Single ring aromatics: i: Toluene, NR(i)=1, ME(i)=1; i: Xylene,        NR(i)=1, ME(i)=2        Fused aromatic rings: i: Indane, NR(i)=1, ME(i)=0; is Tetralin,        NR(i)=1, ME(i)=0;

i: Naphthalene, NR(i)=2, ME(i)=0

Substituents on saturated fused ring: i: 1-methyl-indane and2-methyl-indane (where one methyl group is attached to the five carbonring), NR(i)=1, ME(i)=0Substituents on unsaturated fused ring: i: 4-methyl-indane and5-methyl-indane (where one methyl group is attached to the phenyl ring),NR(i)=1, ME(i)=1; i: dimethyl 2,6-naphthalene, NR(i)=2, ME(i)=2Hence, methyl groups are counted when attached to an aromatic group,e.g., phenyl, and not counted when attached to a full or partial, e.g.,fused, saturated ring for fused-ring compounds having aromatic andsaturated rings.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the application and uses of the embodimentdescribed. Furthermore, there is no intention to be bound by any theorypresented in the preceding background or the following detaileddescription.

The description of the apparatus of this invention is presented withreference to the drawings. FIGS. 2 and 3 are simplified diagrams of thepreferred embodiment of this invention and is not intended as an unduelimitation on the generally broad scope of the description providedherein and the appended claims. Certain hardware such as valves, pumps,compressors, heat exchangers, instrumentation and controls, have beenomitted as not essential to a clear understanding of the invention. Theuse and application of this hardware is well within the skill of theart.

The various embodiments described herein relate to a zero-benzenearomatics process. As shown in FIG. 1, the current design of the process100 for producing no benzene in an aromatics complex. In FIG. 1, thearomatic feed 12 is combined with stream 14 and stream 16 to producestream 18 which is sent to a benzene column 20. The overhead stream 22comprising benzene is sent to a trans-alkylation unit 26 and the bottomsstream 24 is sent to a toluene column. The toluene column overheadstream 30 comprising toluene is sent to a toluene methylation unit 34and the toluene column bottoms stream 32 is sent to anotherfractionation zone 36. The overhead stream from the fractionation zonecomprises mixed xylenes and the bottoms stream 40 comprises heavyaromatics. A stream 42 comprising A9-A10 is sent to the trans-alkylationunit 26. The product stream 14 from the trans-alkylation unit 26 is sentback to be combined with the aromatic feed 12. The product stream fromthe toluene methylation unit 34 is also sent back to be combined withthe aromatic feed 12.

In the example shown in FIG. 2, the aromatic feed 12 has a high methylto phenyl ratio. As shown in FIG. 2, the new design of the process 200for producing no benzene in an aromatics complex. In FIG. 2, thearomatic feed having a high methyl to phenyl ratio 12 is combined withstream 14 and stream 16 to produce stream 18 which is sent to a benzenecolumn 20. The methyl to phenyl ratio of stream 12 may be above about1.5. The overhead stream 22 comprising benzene is sent to atrans-alkylation unit 26 and the bottoms stream 24 is sent to a toluenecolumn. The toluene column overhead stream 30 comprising toluene is sentto a toluene methylation unit 34 and the toluene column bottoms stream32 is sent to another fractionation zone 36. Here, there is an additionstream 31 comprising toluene that is cut from stream 30 and sent to becombined with stream 22 which is being sent to the trans-alkylation unit26. The overhead stream from the fractionation zone comprises mixedxylenes and the bottoms stream 40 comprises heavy aromatics. Stream 42comprising A9-A10 is sent to the trans-alkylation unit 26. The productstream 14 from the trans-alkylation unit 26 is sent back to be combinedwith the aromatic feed 12. The product stream from the toluenemethylation unit 34 is also sent back to be combined with the aromaticfeed 12.

In the example shown in FIG. 3, the aromatic feed 12 has a low methyl tophenyl ratio. As shown in FIG. 3, the new design of the process 300 forproducing no benzene in an aromatics complex. In FIG. 3, the aromaticfeed having a low methyl to phenyl ratio 12 is combined with stream 14and stream 16 to produce stream 18 which is sent to a benzene column 20.Here, there is more benzene sent to the bottoms stream 24. The processconditions of the benzene column 20 are configured so that more benzeneis allowed in the benzene column bottoms. The methyl to phenyl ratio ofstream 12 may be between about 0 and about 1.5. The overhead stream 22comprising benzene is sent to a trans-alkylation unit 26 and the bottomsstream 24 is sent to a toluene column. The toluene column overheadstream 30 comprising toluene and benzene is sent to a toluenemethylation unit 34 and the toluene column bottoms stream 32 is sent toanother fractionation zone 36. The overhead stream from thefractionation zone comprises mixed xylenes and the bottoms stream 40comprises heavy aromatics. Stream 42 comprising A9-A10 is sent to thetrans-alkylation unit 26. The product stream 14 from thetrans-alkylation unit 26 is sent back to be combined with the aromaticfeed 12. The product stream from the toluene methylation unit 34 is alsosent back to be combined with the aromatic feed 12.

While the invention has been described with what are presentlyconsidered the preferred embodiments, it is to be understood that theinvention is not limited to the disclosed embodiments, but it isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims.

EXAMPLES

The following examples listed in Table 1 are intended to furtherillustrate the subject embodiments. These illustrations of differentembodiments are not meant to limit the claims to the particular detailsof these examples.

TABLE 1 Catalyst: 65/35 MWW(SAR~47)/Al2O3 Feed Product Conditions WHSV,hr−1 1.9 1.9 3.8 Catalyst max 287 291 289 temperature, C. Pressure, psig48 295 292 Yield, wt % C1-C4 0.00 0.19 1.07 0.92 Benzene 70.54 39.4049.24 61.17 Toluene 28.03 37.27 33.20 29.35 A8 0.29 12.23 8.92 4.58 A9(excluding indane) 0.12 5.03 2.92 1.23 A10 (excluding 0.03 1.79 1.120.43 methylindanes) A11+ (single ring) 0.06 3.54 3.47 2.28 Other 0.951.84 1.50 1.04 Aromatics Benzene 38.1 24.5 9.9 conversion, Toluene 64.743.2 20.0 mol % Methanol conversion, wt % 94 62 32 Benzene reactivity, %of Toluene 59 57 49 pX/X 45.6 48.2 44.5 oX/X 30.8 37.0 41.2 mX/X 23.614.8 14.2

Table 1 demonstrates that benzene can be converted at high per passconversion in a toluene methylation unit. Non-aromatics are excludedbecause they don't react with methanol at conditions used. Oxygenates(methanol and dimethyl ether) are also excluded, and some components aregrouped together. Benzene conversion is defined as decrease of benzenein product relative to feed (in moles), and toluene conversion isdefined as a sum of A8-A12 in product relative to toluene in feed (alsoin moles). Methanol conversion estimated by counting alkyl groups inaromatics. Benzene “reactivity” is a simple ratio of benzene to tolueneconversion, in %. Generally, benzene reactivity is shown to be at leasthalf of the toluene reactivity in a toluene methylation unit. It isfurther noted that contents of para-xylene in xylene fractions exceedsthe equilibrium concentration of about 24%. It should be noted thatvarious changes and modifications to the presently preferred embodimentsdescribed herein will be apparent to those skilled in the art. Suchchanges and modifications may be made without departing from the spiritand scope of the present subject matter and without diminishing itsattendant advantages.

SPECIFIC EMBODIMENTS

While the following is described in conjunction with specificembodiments, it will be understood that this description is intended toillustrate and not limit the scope of the preceding description and theappended claims.

A first embodiment of the invention is a process for producing nobenzene in an aromatics complex, comprising passing an aromatics streamhaving a low methyl to phenyl ratio to a benzene column to produce abenzene column overhead stream and a benzene column bottoms stream;passing the first portion of benzene column overhead stream to atrans-alkylation unit of A9+ with benzene to produce a trans-alkylationproduct stream and passing the benzene column bottoms stream to atoluene column which produces a toluene column overhead stream and atoluene column bottoms stream; passing the second portion of benzenecolumn overhead to methylation unit to produce methylated productstream; passing the toluene column overhead stream to a methylation unitto produce a methylation product stream; and passing the toluene columnbottoms stream to a fractionation zone to produce an overhead stream, abottoms stream, and a middle boiling fraction, wherein the middleboiling fraction is sent to the trans-alkylation unit of A9+ withbenzene. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph, wherein the toluene methylation unit produces an effluentcontaining xylenes with para-xylene selectivity between 20% and 99.9%within the xylene fraction. An embodiment of the invention is one, anyor all of prior embodiments in this paragraph up through the firstembodiment in this paragraph, wherein the toluene methylation unitproduces an effluent containing xylenes with para-xylene selectivitypreferably between 50% and 99.9% within the xylene fraction. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph,wherein the methyl to phenyl ratio of the aromatic steam is about 0 toabout 1.5. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph, wherein the methyl to phenyl ratio of the aromatic stream isabout 0 to about 1.0. An embodiment of the invention is one, any or allof prior embodiments in this paragraph up through the first embodimentin this paragraph, wherein the methyl to phenyl ratio of the aromaticstream is about 0 to about 0.5. An embodiment of the invention is one,any or all of prior embodiments in this paragraph up through the firstembodiment in this paragraph, wherein the fractionation zone comprises aplurality of fractionation units. An embodiment of the invention is one,any or all of prior embodiments in this paragraph up through the firstembodiment in this paragraph, wherein benzene is allowed to pass to thebottom of the benzene column. An embodiment of the invention is one, anyor all of prior embodiments in this paragraph up through the firstembodiment in this paragraph, wherein toluene and benzene are sent tothe methylation unit. An embodiment of the invention is one, any or allof prior embodiments in this paragraph up through the first embodimentin this paragraph, wherein the portion of the benzene column overheadstream is combined with the toluene column overhead stream before beingsent to the toluene methylation unit. An embodiment of the invention isone, any or all of prior embodiments in this paragraph up through thefirst embodiment in this paragraph, further comprising passing methanolor another alkylation agent to the toluene methylation unit.

A second embodiment of the invention is a process for producing nobenzene in an aromatics complex, comprising passing an aromatics streamhaving a methyl to phenyl ratio of about 0 to about 1.5 to a benzenecolumn to produce a benzene column overhead stream and a benzene columnbottoms stream; passing the first portion of benzene column overheadstream to a trans-alkylation unit of A9+ with benzene to produce atrans-alkylation product stream and passing the benzene column bottomsstream to a toluene column which produces a toluene column overheadstream and a toluene column bottoms stream; passing the second portionof benzene column overhead to methylation unit to produce methylatedproduct stream; passing the toluene column overhead stream to amethylation unit to produce a methylation product stream; and passingthe toluene column bottoms stream to a fractionation zone to produce anoverhead stream, a bottoms stream, and a middle boiling fraction,wherein the middle boiling fraction is sent to the transalkylation unitof A9+ with benzene. An embodiment of the invention is one, any or allof prior embodiments in this paragraph up through the second embodimentin this paragraph, wherein the fractionation zone comprises a pluralityof fractionation units. An embodiment of the invention is one, any orall of prior embodiments in this paragraph up through the secondembodiment in this paragraph, wherein the toluene methylation unitproduces an effluent containing xylenes with para-xylene selectivitybetween 20% and 99.9% within the xylene fraction. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the second embodiment in this paragraph wherein the toluenemethylation unit produces an effluent containing xylenes withpara-xylene selectivity preferably between 50% and 99.9% within thexylene fraction. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the second embodiment inthis paragraph, wherein benzene is allowed to pass to the bottom of thebenzene column. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the second embodiment inthis paragraph, wherein toluene and benzene are sent to the methylationunit. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the second embodiment in thisparagraph, further comprising passing methanol or another alkylationagent to the toluene methylation unit.

Without further elaboration, it is believed that using the precedingdescription that one skilled in the art can utilize the presentinvention to its fullest extent and easily ascertain the essentialcharacteristics of this invention, without departing from the spirit andscope thereof, to make various changes and modifications of theinvention and to adapt it to various usages and conditions. Thepreceding preferred specific embodiments are, therefore, to be construedas merely illustrative, and not limiting the remainder of the disclosurein any way whatsoever, and that it is intended to cover variousmodifications and equivalent arrangements included within the scope ofthe appended claims.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by weight, unless otherwise indicated.

1. A process for reducing benzene production in an aromatics complex,the process comprising: passing an aromatics stream having a methyl tophenyl ratio below about 1.5 to a benzene column to produce a benzenecolumn overhead stream and a benzene column bottoms stream; passing afirst portion of the benzene column overhead stream to atrans-alkylation unit to produce a trans-alkylation product stream;passing the benzene column bottoms stream to a toluene column toproduces a toluene column overhead stream and a toluene column bottomsstream; passing a second portion of the benzene column overhead to amethylation unit; passing the toluene column overhead stream to themethylation unit, wherein the methylation unit produces a methylationproduct stream; passing the toluene column bottoms stream to afractionation zone to produce an overhead stream, a bottoms stream, anda middle boiling fraction; passing the middle boiling fraction from thefractionation zone to the trans-alkylation unit; recycling thetrans-alkylation product stream to the benzene column; and, recyclingthe methylation product stream to the benzene column.
 2. The process ofclaim 1, wherein the methylation unit produces an effluent containingxylenes with para-xylene selectivity between 20% and 99.9% within thexylene fraction.
 3. The process of claim 1, wherein the methylation unitproduces an effluent containing xylenes with para-xylene selectivitypreferably between 50% and 99.9% within the xylene fraction. 4.(canceled)
 5. The process of claim 1, wherein the methyl to phenyl ratioof the aromatic stream is less than about 1.0.
 6. The process of claim1, wherein the methyl to phenyl ratio of the aromatic stream is lessthan about 0.5.
 7. The process of claim 1, wherein the fractionationzone comprises a plurality of fractionation units.
 8. The process ofclaim 1, wherein the benzene column bottoms stream comprises benzene. 9.(canceled)
 10. The process of claim 1, wherein the second portion of thebenzene column overhead stream is combined with the toluene columnoverhead stream before being sent to the methylation unit.
 11. Theprocess of claim 1, further comprising passing methanol or anotheralkylation agent to the toluene methylation unit.
 12. A process forreducing benzene production in an aromatics complex, the processcomprising: separating an aromatics stream having a methyl to phenylratio between about 0 to about 1.5 to a benzene column to produce abenzene column overhead stream and a benzene column bottoms stream;trans-alkylating a first portion of benzene column overhead stream in atrans-alkylation unit to produce a trans-alkylation product stream;separating the benzene column bottoms stream to a toluene column whichproduces a toluene column overhead stream and a toluene column bottomsstream; methylating a second portion of benzene column overhead and thetoluene column overhead stream in a methylation unit, wherein themethylation unit produces a methylation product stream; and separatingthe toluene column bottoms stream in a fractionation zone into anoverhead stream, a bottoms stream, and a middle boiling fraction,wherein the middle boiling fraction is sent to the transalkylation unit;separating the trans-alkylation product stream and the methylationproduct stream in the benzene column.
 13. The process of claim 12,wherein the fractionation zone comprises a plurality of fractionationunits.
 14. The process of claim 12, wherein the methylation unitproduces an effluent containing xylenes with para-xylene selectivitybetween 20% and 99.9% within the xylene fraction.
 15. The process ofclaim 12 wherein the methylation unit produces an effluent containingxylenes with para-xylene selectivity preferably between 50% and 99.9%within the xylene fraction.
 16. The process of claim 12, wherein thebenzene column bottoms stream comprises benzene.
 17. (canceled)
 18. Theprocess of claim 12, further comprising passing methanol or anotheralkylation agent to the toluene methylation unit.